WASHINGTON -- Nine boxes of documents detailing illegal acts by a burglary team set up inside the Nixon White House -- files that historians say could help answer nagging questions about Watergate -- have vanished.

The missing files contained the "hot stuff" about the activities of the "plumbers" unit, said the group's leader, Egil "Bud" Krogh Jr. He was imprisoned for ordering a break-in at the office of Daniel Ellsberg's psychiatrist in 1971.

Federal archivists say they presume that Mr. Krogh's files were merely misplaced when they vanished from the Justice Department in spring 1974, while Richard M. Nixon was still president. But at least one presidential scholar suspects that it's no accident.

"People should rightly believe that they have been stolen or destroyed -- that's my working hypothesis," said Bruce Oudes, editor of the 1989 book, "From the President: Richard Nixon's Secret Files."

Indeed, nearly 19 years after the Watergate break-in, questions about the "third-rate burglary" that cost Mr. Nixon his presidency are resurfacing, with some suggesting that the cover-up never ended.

For example, historians still want to know what the Watergate burglars were after when police caught them at Democratic Party headquarters on June 17, 1972. Their arrests set in motion the investigation and cover-up that ultimately led Mr. Nixon to resign in 1974.

Mr. Nixon has attributed the break-in to overzealousness on the part of his re-election staff, who he said were trying to find out how the Democrats might seek to disrupt the 1972 Republican convention.

But in his account of the break-in, G. Gordon Liddy, who directed the burglary, said the mission was to "find out what [Democratic Party Chairman Lawrence F.] O'Brien had of a derogatory nature about us, not for us to get something on the Democrats."

Mr. Nixon continues to oppose the release of many documents from his presidency that historians say might shed light on the mystery.

Mr. Nixon has challenged the release of some files on the ground that it would violate his right to privacy. Other documents have PTC been kept secret for national security reasons, as defined by the government.

Historians say the missing files and other secret documents and tapes from the Nixon administration could provide the answer to Sen. Howard H. Baker Jr.'s enduring question -- "What did the president know, and when did he know it?"

The "plumbers" included Mr. Liddy and E. Howard Hunt, the men who later orchestrated the break-in at the Democratic Party offices in Washington's Watergate building.

The September 1971 break-in at the Los Angeles office of Mr. Ellsberg's psychiatrist, Dr. Lewis Fielding, presaged the Watergate burglary and marked the start of an illegal espionage unit operating out of the White House.

The burglars were looking for damaging information about Mr. Ellsberg, who had leaked secret documents about how the Vietnam War was conducted -- the Pentagon Papers -- to the New York Times and the Washington Post. Mr. Krogh, Mr. Liddy and other top Nixon aides were convicted of charges stemming from the break-in.

Mr. Krogh's files were supposed to have been made public in 1987. But they were missing from the White House papers

turned over to the National Archives after Mr. Nixon resigned in August 1974. They have never been found.

"Those of us who work in the archives like to see the whole record. And there is a gap. I guess I shouldn't use that word in the Nixon context. Records are missing," said Jim Hastings, former director of the National Archives' collection of Nixon papers.

The importance of the missing files is hard to estimate, but historians say they could be revealing.

"Any missing documents from the plumbers are crucial because they got the marching orders," said historian Roger Morris, who is writing a three-volume biography of Mr. Nixon.

"They were the black-bag guys. They were doing the break-ins that were at the heart of this."

The 230 files contain "the heart of what took place in the special investigation project that was the plumbers group," Mr. Krogh said.

Mr. Krogh said the missing files contain "probably a lot of junk stuff, as any file, but it might disclose names" of co-conspirators in illegal activities.

But he doubted the files would implicate Mr. Nixon in the Ellsberg break-in. "Nothing went to him that I know of," said Mr. Krogh, who left the White House before the Watergate burglary.

Mr. Krogh's files were seized by Watergate special prosecutor Leon A. Jaworski, who made copies of some of them, said Mary Ronan, the archivist of the Watergate special prosecutor's records. Those copied pages would be scattered in the prosecutor's files, she said.

Mr. Krogh's files themselves were transferred in nine boxes to the Justice Department at 2:30 p.m. on April 15, 1974 -- and disappeared. Mr. Oudes asked the FBI to find the papers, but it failed in its search.

"When materials belonging to all the people of the United States are missing, such as the plumbers' papers, we all lose a great deal because it suggests in fact that you can cover up permanently," Mr. Oudes said.

But James A. Wilderotter, the former Justice Department lawyer who signed for Mr. Krogh's files in 1974, thinks they just got lost in the shuffle.

"Nine boxes of files in a governmental universe of 2 trillion boxes are easy to lose, even though they sound now -- 17 years later -- like the family jewels," said Mr. Wilderotter, who is now a lawyer in private practice in Washington.